Messages in this thread Patch in this message | | | Date | Wed, 14 Jan 2004 18:06:38 -0800 | From | Andrew Morton <> | Subject | Re: [2.6.1-mm2] Badness in futex_wait at kernel/futex.c:509 |
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Rusty Russell <rusty@rustcorp.com.au> wrote: > > In message <20040113223612.4fe709d9.akpm@osdl.org> you write: > > Horacio de Oro <hgdeoro@yahoo.com> wrote: > > > > > > Hi! > > > > > > This happen every time I switch from X to console: > > > > > > Badness in futex_wait at kernel/futex.c:509 > > > Call Trace: > > > [futex_wait+434/448] futex_wait+0x1b2/0x1c0 > > > [default_wake_function+0/32] default_wake_function+0x0/0x20 > > > [default_wake_function+0/32] default_wake_function+0x0/0x20 > > > [do_futex+112/128] do_futex+0x70/0x80 > > > [sys_futex+292/320] sys_futex+0x124/0x140 > > > [syscall_call+7/11] syscall_call+0x7/0xb > > > > > > > /* A spurious wakeup should never happen. */ > > WARN_ON(!signal_pending(current)); > > > > (looks at Rusty) > > You were the one who said spurious wakeups shouldn't happen 8) > > This implies that the console code decided to do wake_up_process() on > us. We returned -EINTR to userspace, but there was no signal, which > is odd.
Well this would seem to imply that this task woke up and ran while it was still parked on q.waiters, yes?
> Anyone have any ideas *why*?
Nope. This patch should identify the source of the wakeup. Horacio, could you please add it, see if you get a nice trace? The patch is a bit racy wrt task exit, but it should hold up for long enough to get a trace.
include/linux/sched.h | 1 + kernel/futex.c | 5 ++++- kernel/sched.c | 3 +++ 3 files changed, 8 insertions(+), 1 deletion(-)
diff -puN kernel/futex.c~futex-wakeup-debug kernel/futex.c --- 25/kernel/futex.c~futex-wakeup-debug 2004-01-14 18:01:10.000000000 -0800 +++ 25-akpm/kernel/futex.c 2004-01-14 18:01:10.000000000 -0800 @@ -491,8 +491,11 @@ static int futex_wait(unsigned long uadd * !list_empty() is safe here without any lock. * q.lock_ptr != 0 is not safe, because of ordering against wakeup. */ - if (likely(!list_empty(&q.list))) + if (likely(!list_empty(&q.list))) { + current->flags |= PF_FUTEX_DEBUG; time = schedule_timeout(time); + current->flags &= ~PF_FUTEX_DEBUG; + } __set_current_state(TASK_RUNNING); /* diff -puN include/linux/sched.h~futex-wakeup-debug include/linux/sched.h --- 25/include/linux/sched.h~futex-wakeup-debug 2004-01-14 18:01:10.000000000 -0800 +++ 25-akpm/include/linux/sched.h 2004-01-14 18:01:10.000000000 -0800 @@ -500,6 +500,7 @@ do { if (atomic_dec_and_test(&(tsk)->usa #define PF_SWAPOFF 0x00080000 /* I am in swapoff */ #define PF_LESS_THROTTLE 0x00100000 /* Throttle me less: I clean memory */ #define PF_SYNCWRITE 0x00200000 /* I am doing a sync write */ +#define PF_FUTEX_DEBUG 0x00400000 #ifdef CONFIG_SMP #define SD_FLAG_NEWIDLE 1 /* Balance when about to become idle */ diff -puN kernel/sched.c~futex-wakeup-debug kernel/sched.c --- 25/kernel/sched.c~futex-wakeup-debug 2004-01-14 18:01:10.000000000 -0800 +++ 25-akpm/kernel/sched.c 2004-01-14 18:01:10.000000000 -0800 @@ -770,6 +770,9 @@ static int try_to_wake_up(task_t * p, un } goto out_activate; + if (p->flags & PF_FUTEX_DEBUG) + WARN_ON(!signal_pending(p)); + repeat_lock_task: task_rq_unlock(rq, &flags); rq = task_rq_lock(p, &flags); _
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